1. Field of the Invention
This invention relates generally to a video signal processing circuit that can be used to pre-emphasize high frequency components of an applied signal without allowing excessive voltage excursions of pre-emphasized frequencies and to de-emphasize high frequency components previously emphasized.
2. The Prior Art
It is well known in the art of magnetically recording and reproducing video signals to use the luminance component as a modulating signal with which to modulate the frequency of a carrier. The recording medium of such apparatus is usually, but not necessarily, magnetic tape, but for convenience of description, the apparatus with which the processing circuit of the present invention will be described will be referred to as a VTR, standing for video tape recorder and normally capable of performing both recording and playback functions.
When the VTR is operating in its playback mode, the reproduced FM signal is demodulated to become, as nearly as possible, a replica of the original video signal. It was found in the past that this reproduced video signal had a noise component caused by the frequency modulation and demodulation process and therefore referred to as FM noise, the level of which increased in proportion to the frequency. As a result, pre-enphasis circuits were included in VTRs to reduce the FM noise by emphasizing the high frequency components of the video signal prior to frequency modulating the carrier by means of that signal. In playing back the signal, a de-emphasis circuit was used to attenuate the high frequency components and thus restore the signal substantially to its original waveform while minimizing the FM noise.
It would seem that the S/N (signal-to-noise) ratio of the output signal could be improved by increasing the pre-emphasis and de-emphasis, but the frequency modulation circuits in a VTR operate in relatively narrow band in comparison to the highest frequency luminance signals to be passed through the frequency modulation process. Excessive pre-emphasis causes over modulation by means of the high frequency components, especially if the high frequency components are also of high amplitude. As a result, the amount of pre-emphasis has been limited, with the result that the S/N ratio improvement is also limited.
In a companion application entitled VIDEO SIGNAL PROCESSING CIRCUIT, Ser. No. 758,438, filed Jan. 11, 1977, assigned to the assignee of the present application, a non-linear pre-emphasis circuit was proposed in which an inductor was connected in series with the collector load of an amplifier transistor that has its emitter connected to ground. Non-linearity of response with respect to the high frequency components was accomplished by a pair of back-to-back, or oppositely polarized, diodes connected in parallel with the inductor.
A video signal applied to the base of the transistor was, in effect, differentiated by the load circuit, and the differentiated voltage, corresponding to the high frequency components of the applied video signal, was produced across the inductor. The oppositely polarized diodes had little effect on low amplitude, high frequency components but limited the voltage excursions of high frequency components as the amplitude of such components increased. Thus, the output signal at the collector of the transistor, which included the relatively low frequency components developed across the load resistor and the emphasized high frequency components added to the low frequency components was compressed only in accordance with the amplitude of the high frequency components.
Because of the fact that direct current flows through the diodes in the inductance-type circuits of the companion application, any variation in the direct voltage of the power supply or any change in temperature causes the threshold level E.sub.wd of the compression and expansion portions of the circuit to vary. Since the compression and expansion circuits do not operate simultaneously, such variation of voltage or temperature may not be properly compensated during playback of the recorded information.